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At least 253 records · Page 14

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NASA NTRS1989

Monolithic III-V/Silicon Spatial Light Modulator

Current techniques for growth of device-quality GaAs on silicon substrates enables fabrication of silicon-based version of photodiode-coupled spatial light modulator. Monolithic photodiode-coupled light-modulator array constructed on silicon substrate by growing InAs/GaAs multiple-quantum-wells over silicon PIN diode layer. Intermediate GaAs buffer layer confines attice-misfit dislocations to vicinity of silicon. Use of silicon makes available wider range of auxiliary on-chip signal-processing circuitry for coding and decoding of data, addition or subtraction of brightness levels, spatial reformatting, and rescaling.

Maserjian, Joseph↗

DOE OSTI2016

Preparation of arrays of long carbon nanotubes using catalyst structure

A structure for preparing an substantially aligned array of carbon nanotubes include a substrate having a first side and a second side, a buffer layer on the first side of the substrate, a catalyst on the buffer layer, and a plurality of channels through the structure for allowing a gaseous carbon source to enter the substrate at the second side and flow through the structure to the catalyst. After preparing the array, a fiber of carbon nanotubes may be spun from the array. Prior to spinning, the array can be immersed in a polymer solution. After spinning, the polymer can be cured.

36 MATERIALS SCIENCE↗

DOE OSTI2020

High work function MoO2 back contacts for improved solar cell performance

Improved high work function back contacts for solar cells are provided. In one aspect, a method of forming a solar cell includes: forming a completed solar cell having a substrate coated with an electrically conductive material, an absorber disposed on the electrically conductive material, a buffer layer disposed on the absorber, a transparent front contact disposed on the buffer layer, and a metal grid disposed on the transparent front contact; removing the substrate and the electrically conductive material using exfoliation, exposing a backside surface of the solar cell; depositing a high work function material onto the back side surface of the solar cell; and depositing a back contact onto the high work function material. A solar cell formed by the present techniques is also provided. Yield of the exfoliated device can be improved by removing bubbles from adhesive used for exfoliation and/or forming contact pads to access the metal grid.

14 SOLAR ENERGY↗

DOE OSTI2021

High work function MoO2 back contacts for improved solar cell performance

Antunez, Priscilla D.↗

DOE OSTI2014

Transparent TiO 2 nanotube array photoelectrodes prepared via two-step anodization

Two-step anodization of transparent TiO 2 nanotube arrays has been demonstrated with aid of a Nb-doped TiO 2 buffer layer deposited between the Ti layer and TCO substrate. Enhanced physical adhesion and electrochemical stability provided by the buffer layer has been found to be important for successful implementation of the two-step anodization process. As a result, with the proposed approach, the morphology and thickness of NT arrays could be controlled very precisely, which in turn, influenced their optical and photoelectrochemical properties.

14 SOLAR ENERGY↗

DOE OSTI2024

Ce- and Ni-Codoped Double PrBaMn 2 O 5 Perovskite as a Ceramic SOFC Anode

This study explores the efficacy of cerium introduction, both on its own and in combination with nickel, into PrBaMn 2 O 5+δ (PBM) structures to enhance solid oxide fuel cell (SOFC) anodes. We synthesized Pr 1–x Ce x BaMn 2 O 5+δ compositions for x values of 0.05 (PrCe5) and 0.1 (PrCe10), as well as a nickel-doped variant (PrCe5Ni), assessing their performance under H 2 – 3% H 2 O reducing conditions pertinent to SOFC anode operations. Our findings reveal that the PrCe5 composition exhibits a thermal expansion coefficient (TEC) that not only improves upon that of the Ce-free counterpart but also aligns closely with the TEC standards of prevalent SOFC electrolytes. A notable advancement was achieved with the application of a 3.5 μm gadoliniadoped ceria (GDC) buffer layer through physical vapor deposition, effectively mitigating chemical interactions between PBM-based anodes and yttria-stabilized zirconia (YSZ) electrolytes, a concern highlighted by in situ neutron diffraction analyses. Electrochemical impedance spectroscopy, conducted over 220 h in a H 2 –3% H 2 O atmosphere at 750 °C, demonstrated that the optimal 3.5 μm thickness of the GDC buffer layer significantly minimizes the area-specific resistance (ASR) degradation rate to 0.002 Ω cm 2 /h, markedly outperforming both thinner (1 μm) and thicker (8 μm) GDC layers, which showed higher degradation rates of 0.15–0.2 Ω cm 2 /h due to the diffusion of Ba ions or delamination. Moreover, cerium doping fosters superior microstructural stability and obviates barium diffusion, thereby suggesting an enhanced durability of the doped anodes over their lifespan. Integrating nickel into the PrCe5 structure halved the ASR to 0.5 Ω cm 2 at 750 °C, situating it well within the ideal performance range for SOFC anodes. Furthermore, the enhancement brought about by simultaneously doping PBM with cerium and nickel, which fundamentally relies on the critical contributions of defect chemistry and crystal structure, highlights the significance of these fields in creating sophisticated materials for energy related applications.

25 ENERGY STORAGE↗

DOE OSTI2014

Dielectric relaxation analysis of Pb(Zr{sub 0.54},Ti{sub 0.46})O{sub 3} thin films: Electric field dependence

350 nm-thick Perovskite PbZr{sub 0.54}Ti{sub 0.46}O{sub 3} (PZT) thin films were deposited on Al{sub 2}O{sub 3} substrates by sputtering with and without an additional 10-nm-thick TiO{sub x} buffer layer. X-ray diffraction patterns showed that in presence of TiO{sub x} buffer layer, PZT film was highly oriented along the (111) direction film, whereas the unbuffered, counterpart was polycrystalline. A full wave electromagnetic analysis using a vector finite element method was performed to determine the tunability and the complex permittivity up to 67 GHz. A comparison between the electromagnetic analysis and Cole-Cole relaxation model was proposed. Through an original study of the relaxation time as a function of the electric field, values, such as 2 ps and 0.6 ps, were estimated for E{sub DC} = 0 kV/cm and 235 kV/cm, respectively, and in both cases (111)-PZT and polycrystalline-PZT. The distribution of relaxation times is found to be larger for (111)-PZT film, which is probably related to the film microstructure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2022

Superconducting phase of Ti x O y thin films grown by molecular beam epitaxy

Here we investigate the complex relationship between the growth conditions and the structural and transport properties of Ti x O y thin films grown by molecular beam epitaxy. Transport properties ranging from metallicity to superconductivity and insulating states are stabilized by effectively tuning the O/Ti ratio via the Ti flux rate and the O partial pressure P Ox for films grown on (0001)-Al 2 O 3 substrates at 850° C. A cubic c-TiO 1±δ buffer layer is formed for low O/Ti ratios, while a corundum cr-Ti 2 O 3 layer is formed under higher-oxidizing conditions. Metallicity is observed for c-TiO 1-δ buffer layers. The superconducting γ -Ti 3 O 5 Magnéli phase is found to nucleate on a c-TiO 1-δ buffer for intermediate POx conditions, and an insulator-superconducting transition is observed at 4.5 K (T$^{onset}_{C}$ = 6K) for 85 nm thick films. Strain relaxation of γ -Ti 3 O 5 occurs with increasing film thickness and correlates with a thickness-dependent increase in T C observed for Ti x O y thin films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2023

CMOS-Compatible Ultrathin Superconducting NbN Thin Films Deposited by Reactive Ion Sputtering on 300 mm Si Wafer

We report a milestone in achieving large-scale, ultrathin (~5 nm) superconducting NbN thin films on 300 mm Si wafers using a high-volume manufacturing (HVM) industrial physical vapor deposition (PVD) system. The NbN thin films possess remarkable structural uniformity and consistently high superconducting quality across the entire 300 mm Si wafer, by incorporating an AlN buffer layer. High-resolution X-ray diffraction and transmission electron microscopy analyses unveiled enhanced crystallinity of (111)-oriented δ-phase NbN with the AlN buffer layer. Notably, NbN films deposited on AlN-buffered Si substrates exhibited a significantly elevated superconducting critical temperature (~2 K higher for the 10 nm NbN) and a higher upper critical magnetic field or H c2 (34.06 T boost in H c2 for the 50 nm NbN) in comparison with those without AlN. These findings present a promising pathway for the integration of quantum-grade superconducting NbN films with the existing 300 mm CMOS Si platform for quantum information applications.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Influence of piezoelectric strain on the Raman spectra of BiFeO 3 films deposited on PMN-PT substrates

In this study, BiFeO 3 epitaxial thin films were deposited on piezoelectric 0.72Pb(Mg 1/3 Nb 2/3 )O 3 -0.28PbTiO 3 (PMN-PT) substrates with a conductive buffer layer (La 0.7 Sr 0.3 MnO 3 or SrRuO 3 ) using pulsed laser deposition. The calibration of the strain values induced by the electric field applied on the piezoelectric PMN-PT substrates was realised using X-Ray diffraction measurements. The method of piezoelectrically induced strain allows to obtain a quantitative correlation between strain and the shift of the Raman-active phonons, ruling out the influence of extrinsic factors, such as growth conditions, crystalline quality of substrates, or film thickness. Using the Poisson number for BiFeO 3 one can determine the volume change induced by strain, and therefore the Gr neisen parameters for specific phonon modes.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Specific features of NH{sub 3} and plasma-assisted MBE in the fabrication of III-N HEMT heterostructures

The specific features of how nitride HEMT heterostructures are produced by NH{sub 3} and plasma-assisted (PA) molecular-beam epitaxy (MBE) are considered. It is shown that the use of high-temperature AlN/AlGaN buffer layers grown with ammonia at extremely high temperatures (up to 1150°C) can drastically improve the structural perfection of the active GaN layers and reduce the dislocation density in these layers to values of 9 × 10{sup 8}−1 × 10{sup 9} cm{sup −2}. The use of buffer layers of this kind makes it possible to obtain high-quality GaN/AlGaN heterostructures by both methods. At the same time, in contrast to ammonia MBE which is difficult to apply at T < 500°C (because of the low efficiency of ammonia decomposition), PA MBE is rather effective at low temperatures, e.g., for the growth of InAlN layers lattice-matched with GaN. The results obtained in the MBE growth of AlN/AlGaN/GaN/InAlN heterostructures by both PA-MBE and NH{sub 3}-MBE with an extremely high ammonia flux are demonstrated.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Studies of the Cross Section and Photoluminescence of a GaAs Layer Grown on a Si/Al{sub 2}O{sub 3} Substrate

A GaAs/AlAs/GaAs/AlAs/Ge heterostructure grown on a Si/Al{sub 2}O{sub 3}(1 1-bar 02) substrate is formed and studied. The Ge buffer layer is produced by the “hot wire” technique, whereas the III–V layers are grown by metal–organic vapor-phase epitaxy. The optical quality of the III–V layers is determined by photoluminescence spectroscopy. Structural studies are performed by high-resolution transmission electron microscopy. The elemental composition is determined by energy-dispersive X-ray spectroscopy. In the study, the possibility of growing a single-crystal GaAs layer on a Si/Al{sub 2}O{sub 3} substrate through AlAs/GaAs/AlAs/Ge buffer layers is shown.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Controlling structural phases of Sn through lattice engineering

Topology and superconductivity, two distinct phenomena, offer unique insight into quantum properties and their applications in quantum technologies, spintronics, and sustainable energy technologies. Tin (Sn) plays a pivotal role here as an element because of its two structural phases, α-Sn exhibiting topological characteristics and β-Sn showing superconductivity. Here, we demonstrate precise control of these phases in Sn thin films using molecular beam epitaxy with systematically varied lattice parameters of the buffer layer. The Sn films exhibit either β-Sn or α-Sn phases as the buffer layer's lattice constant varies from 6.10 Å to 6.48 Å, spanning the range from GaSb (like InAs) to InSb. Further, the crystal structures of α- and β-Sn films are characterized by x-ray diffraction and confirmed by Raman spectroscopy and scanning transmission electron microscopy. Atomic force microscopy validates the smooth, continuous surface morphology. Electrical transport measurements further verify the phases: resistance drop near 3.7 K for β-Sn superconductivity and Shubnikov-de Haas oscillations for α-Sn topological characteristics. Density functional theory shows that α-Sn is stable under tensile strain and β-Sn under compressive strain, aligning well with experimental findings. Hence, this study introduces a platform controlling Sn phases through lattice engineering, enabling innovative applications in quantum technologies and beyond.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Characterization of Heteroepitaxial GaAs Films Grown on Si Using Selective Area Nucleation

Selective area growth of GaAs on patterned Si substrates is a potentially low-cost approach to integrate III-V and Si materials for multijunction solar cells. The use of nanoscale openings in a dielectric material can minimize nucleation-related defects and allow thinner buffer layers to be used to accommodate strain and trap defects caused by lattice mismatch between Si and epitaxial III-V layers. We have developed a process to grow coalesced GaAs thin films on Si substrates using buffer layers patterned by soft nanoimprint lithography (SNIL). We use photoelectrochemistry to probe the performance of these films as photovoltaic absorbers, and discuss techniques to improve the material quality of the GaAs epilayer.

14 SOLAR ENERGY↗

DOE OSTI2015

Perpendicularly magnetized (001)-textured D0{sub 22} MnGa films grown on an (Mg{sub 0.2}Ti{sub 0.8})O buffer with thermally oxidized Si substrates

We report the growth of (001)-textured polycrystalline D0{sub 22} MnGa films with perpendicular magnetic anisotropy (PMA) on thermally oxidized Si substrates using an (Mg{sub 0.2}Ti{sub 0.8})O (MTO) buffer layer. The ordered D0{sub 22} MnGa film grown at the optimum substrate temperature of 530 °C on the MTO buffer layer shows PMA with magnetization of 80 kA/m, PMA energy density of 0.28 MJ/m{sup 3}, and coercivity of 2.3 T. The scanning transmission electron microscope analysis confirms the formation of a highly (001)-textured structure and the elementally sharp interfaces between the MTO layer and the MnGa layer. The achieved D0{sub 22} MnGa PMA films on an amorphous substrate will provide the possible pathway of integration of a Mn-based PMA film into Si-based substrates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2019

Photoelectric Properties of GaN Layers Grown by Plasma-Assisted Molecular-Beam Epitaxy on Si(111) Substrates and SiC/Si(111) Epitaxial Layers

The photoelectric properties of GaN/SiC/Si(111) and GaN/Si(111) heterostructures grown by plasma-assisted molecular-beam epitaxy under the same growth conditions on identical silicon substrates, but with different buffer layers, are experimentally investigated. The GaN/SiC/Si(111) structure is formed on a Si substrate with the SiC buffer layer grown by a new atom-substitution technique and the GaN/Si(111) structure, on a Si substrate subjected to pre-epitaxial plasma nitridation. The significant effect of carbon-vacancy clusters contained in the SiC layer on the growth of the GaN layer and its optical and photoelectric properties is found. It is experimentally established that the GaN/SiC/Si(111) heterostructure has a higher photosensitivity than the GaN/Si(111) heterostructure. In the GaN/SiC/Si(111) heterostructure, the coexistence of two oppositely directed p–n junctions is observed. One p–n junction forms at the SiC/Si interface and the other, at the GaN/SiC interface. It is shown that the occurrence of an electric barrier in the GaN/Si(111) heterostructure at the GaN/Si(111) heterointerface is caused by the formation of a thin silicon-nitride transition layer during pre-epitaxial plasma nitridation of the Si(111) substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Monolithic integration of rare-earth oxides and semiconductors for on-silicon technology

Several concepts of integration of the epitaxial rare-earth oxides into the emerging advanced semiconductor on silicon technology are presented. Germanium grows epitaxially on gadolinium oxide despite lattice mismatch of more than 4%. Additionally, polymorphism of some of the rare-earth oxides allows engineering of their crystal structure from hexagonal to cubic and formation of buffer layers that can be used for growth of germanium on a lattice matched oxide layer. Molecular beam epitaxy and metal organic chemical vapor deposition of gallium nitride on the rare-earth oxide buffer layers on silicon is discussed.

36 MATERIALS SCIENCE↗

DOE OSTI2017

High performance top-gated multilayer WSe2 field effect transistors

In this paper, high performance top-gated WSe 2 field effect transistor (FET) devices are demonstrated via a two-step remote plasma assisted ALD process. High-quality, low-leakage aluminum oxide (Al 2 O 3 ) gate dielectric layers are deposited onto the WSe 2 channel using a remote plasma assisted ALD process with an ultrathin (~1 nm) titanium buffer layer. The first few nanometers (~2 nm) of the Al 2 O 3 dielectric film is deposited at relatively low temperature (i.e. 50 °C) and remainder of the film is deposited at 150 °C to ensure the conformal coating of Al 2 O 3 on the WSe 2 surface. Additionally, an ultra-thin titanium buffer layer is introduced at the WSe2 channel surface prior to ALD process to mitigate oxygen plasma induced doping effects. Excellent device characteristics with current on–off ratio in excess of 106 and a field effect mobility as high as 70.1 cm 2 V –1 s –1 are achieved in a few-layer WSe 2 FET device with a 30 nm Al 2 O 3 top-gate dielectric. With further investigation and careful optimization, this method can play an important role for the realization of high performance top gated FETs for future optoelectronic device applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗